The present invention relates to radiotelephone communication systems and more particularly to a method and apparatus for detecting errors in a radiotelephone communications switching unit to base station PCM link, and for handling communications with a mobile unit in the event of a loss of a communications block on the PCM link.
A cellular telephone communications system such as the European Global System for Mobile Communication (GSM) mobile telephone system may include elements as shown in FIG. 1a. A base transceiver station (BTS) 103 is connected to a Base Station Controller (BSC) 101 by means of a Pulse Code Modulation (PCM) link 107. The base transceiver station 103 includes a speech coder-decoder (codec) 109. In the past, digitized speech signals have been transmitted by the BSC 101 at a rate of 64 kilobits per second (kbps) to the base transceiver station 103 by means of the PCM link 107. Once inside the base transceiver station 103, a speech codec 109 transforms the digitized speech into blocks of parameters having fewer bits than the original, thus enabling the speech to be transmitted to the mobile station 105 at a lower bit rate than is possible for the original speech samples. The encoded speech signals are converted back into the original speech samples by a speech decoder (not shown) located inside the mobile station 105.
Recently, the design of systems, such as the GSM system, have been modified as shown in FIG. 1b so that the speech codec 109' is located within the BSC 101' instead of in the base transceiver station 103'. The speech codec 109 may accept, for example, 160 speech samples for each 20 msec of speech, and produce therefrom blocks of encoded speech having a lower bit rate than the original speech. This produces the advantage of allowing transmission over the PCM link 107' of more speech between the base transceiver station 103' and the BSC 101' than was possible before.
The base transceiver station 103' channel encodes, modulates, and transmits each received block of coded speech to the mobile station 105' by means of a radio link. In addition to transmitting coded speech to the mobile station 105', the base transceiver station 103' may also transmit control information generated by a control part (not shown) of the base transceiver station 103'. As previously mentioned, the mobile station 105' also contains a codec (not shown) for reconstructing the original speech from the received blocks of coded speech.
Transmission of speech from the mobile station 105' to the BSC 101' works in a similar way. The mobile station 105' generates and transmits modulated, channel encoded blocks of coded speech to the base transceiver station 103'. The base transceiver station 103' performs demodulation and channel decoding on the received blocks, and uplinks the coded speech to the BSC 101' by means of the PCM link 107'. Should the base transceiver station 103' determine that it has received control information rather than encoded speech from the mobile station 105', then this control information is routed to the control part of the base transceiver station 103 '.
One characteristic of the system as described above is the introduction of errors into the encoded speech blocks due to their transmission over the PCM link 107'. When the speech codec was located in the base transceiver station 103 instead of the BSC 101 ', there was little need for fault handling on a PCM link because the quality of the received speech is not seriously degraded by the presence of such introduced errors. However, with the speech codex 109' located in the BSC 101 ', the speech on the PCM link 107' is compressed into fewer bits. As a result, any errors introduced during transmission on the PCM link 107' seriously effect the quality of the speech after it is decoded.
However, prior systems transmitted encoded speech parameters without any mechanism for detecting errors introduced by the PCM link 107'. For example, in one well known system, the base transceiver station 103' is able to detect, by means of poor received signal quality, the occurrence of errors in speech received from the mobile station 105'. Because the speech received by the base transceiver station 103' from the mobile station 105' is encoded, the detected errors can seriously degrade the quality of the received speech. Consequently, in this well known system, a single bit, called the Bad Frame Indicator (BFI) is sent by the base transceiver station 103' to the BSC 101' along with the block of speech which has been decoded by the base transceiver station 103. When the BSC 101' detects the BFI being set (indicating the presence of errors introduced on the radio channel between the mobile station 105' and the base transceiver station 103'), it discards the received block of speech and instead passes information from the previously received block to the public telephone system. The rationale for doing this is that the previously received speech block is likely to closely resemble what the present speech block should look like, so that speech quality will be maintained.
However, this well known system, which includes the BFI bit, is incapable of detecting errors that are introduced by transmission of speech on the PCM link 107'. As mentioned earlier, this was not critical when the speech codec 109 was located in the base transceiver station 103, since errors introduced into unencoded speech did not seriously degrade the quality of that speech. However, with the codec 109' located in the BSC 101 ', the detection and subsequent handling of errors introduced by transmission on the PCM link 107' is now quite important.